Symmetrical off-set relay circuit utilizing a non-linear difference amplifier



Sept. 4, 1962 W. L. LOG SYMMETRICAL OFF-SET RELAY CIRCUIT UTILIZING A NON-LINEAR DIFFERENCE AMPLIFIER Filed Feb. 28, 1961 SLOPE CONTROLLED BY R T 3 INVENTOR. 1 L 3 WILSON L LOG N BY 1 I ATTORNEY EZ sXMKWJ' AGEN T United States Patent SYMMETRHCAL OFF-SET RELAY CIRCUIT UTILIZ- ING A NON-LINEAR DKFFERENCE AMPLIFIER Wilson L. Logan, Lindenwold, N J assignor to the United States of America as represented by the Secretary of the Air Force Filed Feb. 28, 1961, Ser. No. 92,425 3 Claims. (Cl. 328-142) The purpose of this invention is to provide a circuit having a low voltage gain for input voltages below a predetermined threshold level with the gain rapidly changing to a relatively high value as the input voltage passes through the threshold level. The circuit may be used, for example, to energize a monitor relay. In this case the relay is insensitive to voltages below the threshold but a slight voltage increase above the threshold will produce positive actuation.

Briefly, the circuit comprises a pair of cathode copuled push-pull stages with the cathode coupling controlled by Zener diodes. A more detailed description of the invention will be given with reference to the specific embodiment thereof shown in the accompanying drawing in which FIG. 1 is a schematic diagram of a relay circuit in accordance with the invention,

FIG. 2 shows the characteristic of the circuit of FIG. 1, and

FIG. 3 illustrates the characteristic of a Zener diode.

Referring to FIG. 1, input voltage E is applied between the grid of section A of dual triode 1 and ground. Positive voltage from direct current source 2 is applied to the anode of section A through anode resistance R and the cathode of this section is connected to the grounded negative side of source 2 through cathode resistor R In a similar manner, the anode-cathode path of section B is connected in shunt to source 2 by means of anode resistor R and cathode resistor R R and R may be equal to R and R respectively. Section A is coupled to section B by grounding the grid of section B and connecting the cathode of section A to that of section B through a series circuit comprising oppositely poled Zener diodes CR and CR and gain control rheostat R The load or output circuit, in this case the coil K of a relay, is connected between the anodes of the two sections, the output voltage being the difierence between the two anode voltages.

FIG. 3 illustrates the characteristic of a Zener diode. As is well known, as the back voltage E across a crystal diode is increased from zero the back current I increases very slowly, or, in other words, the incremental impedance dE /dl of the diode in the back direction is very large until at a value of back voltage 2, known as the Zener breakdown voltage, the incremental impedance suddenly decreases to a relatively low value and any further increase in back voltage causes a large increase in back current. The Zener breakdowns of crystal diodes CR and CR are utilized in the circuit of FIG. 1 to produce the desired sharp increase in gain at the threshold voltage, i.e. the value of E at which breakdown occurs. The operation of FIG. 1 is as follows:

When E =O the space currents of sections A and B are equal with the result that the anode potentials of the two sections are equal and also the cathode potentials of the sections are equal. Consequently, no voltage is applied to K or across the series circuit CR R -CR As E increases from zero in the positive direction the cathode potential of section A follows that of the grid due to R As the cathode potential rises a voltage is applied to CR in the back direction. For values of E less than the value E for which the CR back voltage is less than e (FIG. 3), this diode has a very high impedance and therefore substantially no voltage is coupled from the cathode of section A to the cathode of section B. Consequently, the anode voltage of section B remains substantially at its quiescent value. Further, because of the relatively low gain of section A due to the negative feedback produced by R the fall in potential of the anode of section A is slight and only a small voltage insufiicient to operate the relay is developed across K The low gain of the circuit for values of E less than E is illustrated by section 3 of the characteristic shown in FIG. 2.

When E reaches the value E the cathode potential of section A has reached the value where, as stated above, the back voltage across CR equals the Zener breakdown voltage 2. Beyond this point the impedance of CR rapidly changes to a low value and the overall gain of the circuit, as represented by the ratio Ek /E rapidly changes to a much higher value as represented by section 4 of the characteristic in FIG. 2. The fall in the impedance of CR increases the gain of the circuit in two ways: (1) by increasing the coupling between the cathode of section A and the cathode of section B to a ratio approaching R /(R =+R and (2) by reducing the impedance between the cathode of section A and ground, thus reducing the negative feedback in this section and increasing its gain. Consequently, as E increases beyond E in the positive direction the potential of the anode of section A falls more rapidly than before and, in addition, this is accompanied by a rise in the potential of the anode of section B, which results from the rise in the potential of the cathode of section B due to its greatly increased coupling to the cathode of section A. It will be noted that for positive values of E the presence of CR may be disregarded since it is conducting in the forward direction and therefore has a very low impedance.

The operation of the circuit as E increases from zero in the negative direction is similar to that described above for positive values except that, in this case, Zener breakdown occurs in CR while CR always has forward conduction and therefore a low impedance. Negative operation also reverses the polarity of the output voltage E since increasing negative values of E cause the anode potential of section A to rise and the anode potential of Section B to fall. Parts 3 and 4' of the curve in FIG. 2 illustrate operation of the circuit in the negative region. The dotted lines in FIG. 2 may represent the relay actuating voltage.

The gain of the circuit, i.e. the steepness of sections 4 and 4' (FIG. 2), may be controlled by rheostat R which controls the cathode coupling ratio R.,/ (RH-R after Zener breakdown occurs. The value of the voltage E at which the gain change occurs, can be controlled by selecting Zener diodes of suitable breakdown voltage.

I claim:

1. A circuit having a gain that changes abruptly at a predetermined input signal magnitude, comprising: first and second amplifying devices each having an input circuit and an output circuit, means for applying said input signal to the input circuit of said first amplifying device, a coupling between the output circuit of said first amplifying device and the input circuit of said second amplifying device, said coupling being so phased that the output signal of said second device is opposite in phase to that of said first device, said coupling also containing a Zener diode poled to pass the coupled signal in the reverse direction, and a load circuit connected to said output circuits to receive a voltage proportional to the algebraic difference of their output signals.

2. A circuit having a gain that changes abruptly at a predetermined input signal magnitude, comprising: first and second vacuum tubes each having an anode, a cathode and a control grid, means for applying said input signal between the grid of said first tube and a point of reference potential, a source of direct current, separate anode impedances connected between the anodes of said tubes and the positive terminal of said source, separate cathode impedances connected between the cathodes of said tubes and said point of reference potential, means connecting the negative terminal of said source and the grid of said second tube to said point of reference potential, a load device connected between said anodes, and a Zener diode connected between said cathodes and poled so that the potential developed thereacross as the result of said input signal is in the reverse direction of the diode.

3. A circuit having a gain that changes abruptly at 20 a predetermined input signal magnitude, comprising: first and second vacuum tubes each having an anode, a

4 cathode and a control grid, means for applying said input signal between the grid of said first tube and a point of reference potential, a source of direct current, separate anode impedances connected between the anodes of said tubes and the positive terminal of said source, separate cathode impedances connected between the cathodes of said tubes and said point of reference potential, means connecting the negative terminal of said source and the grid of said second tube to said point of reference potential, a load device connected between said anodes, and a pair of oppositely poled Zener diodes and a variable resistance connected in series between said cathodes.

References Cited in the file of this patent UNITED STATES PATENTS Gilbert Nov. 18, 1958 OTHER REFERENCES 

